Electronic copying machines capable of making a hard copy of notes, illustrations or the like written on a writing board itself or on a sheet placed on or tacked to the writing board are known, wherein an image line sensor (hereinafter referred to simply as a line sensor) moves stepwise in a transverse direction in the focal plane of a taking lens to scan an image formed by the taking lens and to generate at each step image signals representative of a line of pixels of the image, based on which a printer prints out the image on a thermographic or other type copy paper sequentially from line to line. The line sensor has a great number of pixels arranged in a line for photoelectrically converting incident light into signal charges and storing the signal charges, and a shift register for transferring the signal charges as serial image signals.
Before picking up the image by the line sensor in such a conventional electronic copying machine, an average brightness of the whole surface to be printed is detected by a photosensor disposed at the front of the electronic copying machine; and according to the average brightness the charge storage time of the line sensor is adjusted so as to control the exposure for making a good copy.
If, however, an image to be printed has very bright or dark areas, for example if an image is bright on the right side and dark on the left side, then the charge storage time determined in accordance with the detected average brightness of the whole surface would be too long for the bright area and too short for the dark area, resulting in a hard copy of poor quality.